Co-digestion of sewage sludge and crude glycerol for on-demand biogas production

• Crude glycerol was as readily biodegradable as pure glycerol.
• Glycerol addition to sewage sludge at low doses did not cause any negative impact.
• Intermittent glycerol addition could be used for on-demand biogas production.
• The findings were confirmed with BioWin simulation and pilot experiments.

Pilot scale experiments, biomethane potential (BMP) evaluation, and BioWin simulations were conducted to evaluate the intermittent co-digestion of sewage sludge and crude glycerol for on-demand biogas production. BMP tests revealed that both pure and crude glycerols were readily biodegradable. BioWin simulations showed that intermittent glycerol injection at a high (3% v/v) dose might lead to an increase in chemical oxygen demand of the digestate. Results from the pilot scale experiments confirmed that intermittent injection of crude glycerol at both low (0.63% v/v) and high (3%v/v) doses could be used for on-demand biogas production to match the daily fluctuation in energy consumption at a typical wastewater treatment plant. However, in terms of additional biogas production per volume of added glycerol, the lower dose (0.63% v/v) was more effective. The additional methane yield (at the glycerol dose of 0.63% v/v) was 1.3 m3 per litre of crude glycerol. This value obtained from the pilot scale experiment was higher than that from the BMP test but lower compared to that predicted from the BioWin simulation.